Make sure you have the latest app version and open the neon app. Go to «Profile», select the tile «Plans & extensions», and choose «neon duo». Then you confirm your details and can invite your partner with whom you would like to share a joint account. More about the joint account. 

New to neon? To open a joint account, you and your partner or roommate each need a personal neon account (free, plus, global, or metal). And you must live in the same household.